The present invention relates to a constant current circuit for supplying a constant current to a power source.
A conventional constant current circuit is illustrated in FIG. 1. The operation principle will be hereinafter described in conjunction with FIG. 1. When the current flowing through a load 1 is represented by I.sub.ref, a voltage V.sub.r produced across a resistor 3 having a resistance value of R is expressed by the following equation. EQU V.sub.R =R.I.sub.ref1 (1)
The voltage V.sub.R produced across the resistor 3 represented by the equation (1) is compared with the output voltage V.sub.ref of a constant voltage circuit 5 in a comparator 4 as illustrated in FIG. 1, and a constant current insulated gate field effect transistor 2 is driven by the output therefrom. In this case, the transistor 2 is turned on when V.sub.ref is larger than V.sub.R and is turned off when V.sub.ref is smaller than V.sub.R. That is, the constant current insultated gate field effect transistor is operated in such a way that V.sub.R is equal to V.sub.ref. Therefore, the constant current I.sub.ref1 produced by the constant current circuit as illustrated in FIG. 1 will be expressed as follows. EQU I.sub.ref1 =V.sub.ref /R (2)
As clearly understood from the equation (2), the conventional constant current circuit has the following disadvantages.
1. Since the comparator and the constant voltage circuit 5 are required, the circuit is complex and cannot be operated with less power.
2. The resistor 3 cannot be fabricated with less dispersion by the present integrated circuit technique so that the circuit is not suitable for integrated circuit.
An object of the present invention is to provide a constant current circuit, in which the defects in the prior art are removed, the dispersion of the constant current I.sub.ref1 is smaller, and the number of the elements is reduced so as to easily fabricate as an integrated circuit.